<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        body{
            background: url(123/1.png);
        }
    </style>
</head>
<body>
    <select name="" id="sele">
        <option value="1">背景1</option>
        <option value="2">背景2</option>
        <option value="3">背景3</option>
        <option value="4">背景4</option>
        <option value="5">背景5</option>
    </select>
    <script>
        var sele = document.getElementById("sele");
        sele.onchange = function(){
            document.body.style.background = "url(123/"+this.value+".png)"
            // alert(this.value)
            
            // switch(this.value){
            //     case "1":document.body.style.background = "url(123/1.png)"
            //     break;
            //     case "2":document.body.style.background = "url(123/2.png)"
            //     break;
            //     case "3":document.body.style.background = "url(123/3.png)"
            //     break;
            //     case "4":document.body.style.background = "url(123/4.png)"
            //     break;
            //     case "5":document.body.style.background = "url(123/5.png)"
            //     break; 
            // }

        }
    </script>
</body>
</html>